The SOHO–Stellar Connection

The solar–stellar connection bridges the daytime and nighttime communities; an essential link between the singular, but detailed, views of our Sun, and the broad, but coarse, glimpses of the distant stars. One area in particular – magnetic activity – has profited greatly from the two way traffic in ideas. In that spirit, I present an evolutionary context for coronal activity, focusing on the very different circumstances of low-mass main-sequence stars like the Sun, compared with more massive stars. The former are active mainly very early in their lives, whereas the latter become coronal only near the end of theirs, during the brief incursion into the cool half of the Hertzsprung–Russell diagram as yellow, then red, giants. I describe tools at the disposal of the stellar astronomer; especially spectroscopy in the ultraviolet and X-ray bands where coronae leave their most obvious imprints. I compare HST STIS spectra of solar-type dwarfs – Dor (F7 V), an active coronal source, and Cen A (G2 V), near twin of the Sun – to the SOHO SUMER UV solar atlas. I also compare the STIS line profiles of the active coronal dwarf to the corresponding features in the mixed activity hybrid chromosphere bright giant TrA (K2 II) and the archetype non-coronal red giant Arcturus ( Boo; K2 III). The latter shows dramatic evidence for a cool absorber in its outer atmosphere that is extinguishing the hot lines (like Siiv 1393 and Nv 1238) below about 1500 Å; the corona of the red giant seems to lie beneath its extended chromosphere, rather than outside as in the Sun. I present an early taste of the moderate resolution spectra we can expect from the recently launched Chandra X-ray Observatory (CXO), and contemporaneous STIS high resolution UV measurements of the CXO calibration star Capella ( Aur; G8 III + G1 III). Last, I describe preliminary results from a May 1999 observing campaign involving SOHO SUMER, TRACE, and the Kitt Peak Infrared Imaging Spectrometer (IRIS). The purpose was to explore the dynamics of the quiet solar atmosphere through the key magnetic transition zone that separates the kinetically dominated deep photosphere from the magnetically dominated coronal regime. Linking spatially and temporally resolved solar phenomena to properties of the average line shapes (widths, asymmetries, intensity ratios, and Doppler shifts) is a crucial step in carrying physical insights from the solar setting to the realm of the distant stars.     

Dependence of radio emission in large Hα flares 1967–1970 upon the orientation of the local solar magnetic field

During 1967–1970, the greatness of 90 large flares (H importance 2) was influenced by the orientation of the large-scale ( 100 000 km) magnetic field structure over the flare site. Although the average X-ray and optical emissions are only slightly larger for flares with their overlying fields directed southward, as opposed to northward, the meter-wave-length prompt flux maxima are, on average, an order of magnitude greater for the flares with southward oriented magnetic fields. There is a comparable, but possibly smaller difference in the 10 cm- fluxes. We therefore conclude that, during this period, the orientation of the overlying magnetic field affects the amount of electromagnetic flare energy radiated promptly in the corona (10 cm- and m-), relative to that radiated in the chromosphere (X-ray and optical). We demonstrate that this statistical effect shows some variability in degree during the period, although the trend is consistent throughout.     

A comparison between photospheric and chromospheric quiet-Sun magnetograms

Photospheric (Fei 5324.19 Å line) and chromospheric (H line) magnetic fields in quiet-Sun regions have been observed in the solar disk center by using the vector video magnetograph at Huairou Solar Observing Station of Beijing Astronomical Observatory. Observational results show that the quiet-Sun magnetic elements in the solar photosphere and chromosphere present similar magnetic structures. Photospheric and chromospheric magnetograms show corresponding time variations. This suggests that the magnetic fields in quiet-Sun regions present different 3-D magnetic configurations compared to those in solar active regions.     

THE RESONANT ABSORPTION OF 5-MIN OSCILLATIONS IN THE SOLAR ATMOSPHERE

The model calculations of 5-min solar oscillations are performed with consideration for the presence of canopy magnetic field in the solar chromosphere. It is shown that the occurrence of Alfvén resonances for 5-min oscillations in the solar chromosphere leads on the one hand to some change of the 5-min oscillation frequencies (up to a few µHz), and on the other hand to the heating of the chromosphere. The acoustic energy flux incoming to the chromosphere is of order 1 × 10 5 erg cm-2 s-1.     

Soft X-ray observations of large-scale coronal active region brightenings

One-hundred fifty-six large-scale enhancements of X-ray emission from solar active regions were studied on full-disk filterheliograms to determine characteristic morphology and expansion rates for heated coronal plasma. The X-ray photographs were compared with H observations of flares, sudden filament disappearances, sprays and loop prominence systems (LPS). Eighty-one percent of the X-ray events were correlated with H filament activity, but only forty-four percent were correlated with reported H flares. The X-ray enhancements took the form of loops or arcades of loops ranging in length from 60 000 km to 520 000 km and averaging 15 000 km in width. Lifetimes ranged from 3 hr to >24 hr. Event frequency was 1.4 per day. X-ray loop arcades evolved from sharp-edged clouds in cavities vacated by rising H filaments. Expansion velocities of the loops were 50 km s^-1 immediately after excitation and 1–10 km s^-1 several hours later. These long-lived loop arcades are identified with LPS, and it is suggested that the loops outlined magnetic fields which were reconnecting after filament eruptions. Another class of X-ray enhanced loops stretched outside active regions and accompanied sprays or lateral filament ejections. H brightenings occurred where these loops intersected the chromosphere. Inferred excitation velocities along the loops ranged between 300 and 1200 km s^-1. It is suggested that these loops outlined closed magnetic fields guiding slow mode shocks from flares and filament eruptions.     

Magnetic reconnection in high-temperature plasma of solar flares

Quasi-steady high-temperature current sheets are an energy source during the main or hot phase of solar flares. Such sheets are shown to be stabilized with respect to the tearing instability by a small transverse component of magnetic field existing in the sheets.     

Magnetic-cloud chamber effects behind flare-generated shock waves

A mechanism explaining the generation of the helium-enriched plasma-condensation colud (HAE-events) behind the front of shock waves associated with mass-ejecting flares is presented. The mechanism is based on the occurence of physical conditions, analogous to those in a Wilson cloud chamber in a magnetic field, behind the front of a flare-generated shock wave propagation out into interplanetary space. Consequently, if the solar atmosphere above the flare active region is saturated with ejected helium plasma, conditions are created for the forming of the helium-enriched plasma-condensation colud in the temperature-depressed region behind the shock wave front.     

Permanent changes in filaments near solar flares

High resolution H images obtained before and after 57 importance 1N or larger flares have been examined for changes in the magnetic fields (B ) transverse to the line of sight. It was assumed that H chromospheric structures outline B . In 37% of the cases, there was a reconfiguration of segments of filaments or of chromospheric fibrils. Examination of data from 21 non-flare intervals shows such changes in 24% of cases. When changes of any kind, including total disappearance and length changes, are included, the proportions for flare and non-flare intervals increase to 58% and 52%, respectively. It is concluded that flares do not cause enduring magnetic field changes in the chromosphere.     

Theoretical model of flares and prominences

A theoretical model of flare which explains observed quantities in H, EUV, soft X-ray and flare-associated solar wind is presented. It is assumed that large mass observed in the soft X-ray flare and the solar wind comes from the chromosphere by the process like evaporation while flare is in progress. From mass and pressure balance in the chromosphere and the corona, the high temperature in the soft X-ray flare is shown to be attained by the larger mass loss to the solar wind compared with the mass remained in the corona, in accord with observations. The total energy of 10³² erg, the electron density of 10^13.5 cm^–3 in H flare, the temperature of the X-ray flare of 10^7.3K and the time to attain maximum H brightness (600 s) are derived consistent with observations. It is shown that the top height of the H flare is located about 1000 km lower than that of the active chromosphere because of evaporation. So-called limb flares are assigned to either post-flare loops, surges or rising prominences.The observed small thickness of the H flare is interpreted by free streaming and/or heat conduction. Applications are suggested to explain the maximum temperature of a coronal condensation and the formation of quiescent prominences.     

A kinematic model of a solar flare

Hyder advocated the idea that the optical (H) flares can be identified with the response of the solar chromosphere to an infalling material stream resulting from the disparition brusque of a prominence. Since some flares are observed without any apparent association with infalling streams, in this paper we examine the possibility of identifying the optical flare with the response of the chromosphere to a supersonic disturbance, i.e., a shock, propagating downward. The undisturbed chromosphere is represented by the Harvard-Smithsonian Reference Atmosphere and the evolution of the shock is evaluated with the use of the CCW (Chisnell, Chester, Whitham) approximation based on the theory of characteristics. It is shown that the chromosphere is heated by the shock and that radiation is enhanced, and that the enhanced radiation terminates the shock around the height of the temperature minimum. Numerical results obtained and possible future improvements of this type of study are discussed.The National Center for Atmospheric Research is sponsored by the National Science Foundation.     

Electrostatic Wave Instability and Plasma Radiation from Coronal Loops

Polarization properties of solar and stellar radio emission require, in some cases, emission below the third or fourth coronal electron gyro level, < 3,_c; 4, _c. In the context of plasma radiation, the source parameters should be such that the intermediate magnetic field condition 1 < _p ² / _c ² < 3 is satisfied. Supposing this condition, we investigate the generation of electrostatic waves in a warm background plasma with a high-energy component of magnetically trapped electrons. We invoke the conversion of upper-hybrid waves and Bernstein waves into electromagnetic radiation as being responsible for intense radio emission from a coronal magnetic loop. Moreover, odd-half harmonic emissions in the solar radio spectrum as well as the o-mode polarization at the second harmonic of the plasma frequency are natural consequence of this proposed model.     

Solar soft X-rays and solar activity

Soft solar X-rays (8 gl 12 Å) were observed from OSO-III. An analysis of the X-ray enhancements associated with 165 solar flares revealed that there is a tendency for a weak soft X-ray enhancement to precede the cm- burst and H flare. The peak soft X-ray flux follows the cm- peak by about 4 min, on the average. Additionally, it was found that flare-rich active centers tend to produce flares which are stronger X-ray and cm- emitters than are flares which take place in flare-poor active centers.     

Hα line profile observations of solar flares with high temporal resolution

H line profile observations of solar flares with high temporal resolution are an important tool for the analysis of the energy transport mechanism from the site of the flare energy release to the chromosphere. A specially designed instrument (imaging spectrograph) allows two-dimensional imaging of an active region simultaneously in 15 spectral channels along the H line profile with a temporal resolution of 5.4 s. Two flares have been observed in November 1982. The first one shows H signatures which one would typically expect in the case of explosive chromospheric evaporation produced by massive injection of non-thermal electrons. The observations of the other flare indicate that the heating of the upper chromosphere is dominated by thermal conduction, although during the impulsive hard X-ray burst there are also signatures of heating by non-thermal electrons.     

Solar flare activity and the structure of the coronal neutral line

In this study we analyse the positions of major flares from 1978 and 1979, with respect to the magnetic structure of the solar corona, as described by a potential field model. We find that major flares exhibit no strong association with the neutral line at the chromospheric level. However, when we calculate the neutral line's position at higher and higher altitudes in the corona, we find that major flares show an increasing tendency to be found close to these high-altitude coronal neutral lines. The correlation between flares and higher-altitude coronal neutral lines reaches a maximum at an altitude of 0.35R , and thereafter decreases as the neutral line is moved out to the source surface at an altitude of 1.50R . This indicates that major flares are strongly associated with coronal structure at the 0.35R level ( 250 000 km) - an altitude surprisingly high in the corona. This reinforces the idea that flares are associated with large-scale coronal magnetic fields and also indicates that the region of coronal magnetic topology important to solar flare processes may be larger than previously thought.     

ELECTRIC CURRENT HELICITY IN 40 ACTIVE REGIONS IN THE MAXIMUM OF SOLAR CYCLE 22

Using vector magnetograms of 40 active regions (ARs) of the maximum of solar cycle 22, we calculated the imbalance _h (over the AR area) of the current helicity h_c B_z ( × B)_z in the photosphere. In 82.5% of the cases the predominant current helicity was negative (_h < 0) in the northern hemisphere and positive (_h > 0) in the southern hemisphere. Thus, the predominance of counter-clockwise (clockwise) vortices in the northern (southern) hemisphere seems to be valid not only for unipolar spots with obvious vortex structure (Hale, 1927; Richardson, 1941; McIntosh, 1979; Ding, Hong, and Wang, 1987) but also for ARs of different types. The forces of rotation of the Sun (Coriolis force and/or differential rotation) seem to take effect in the twisting of various magnetic structures.     

The acceleration and propagation of solar cosmic rays as deduced from the relative abundance of protons to helium nuclei

Except for protons, the chemical composition of solar cosmic rays is very similar to the abundance of the elements at the photosphere of the Sun. If we consider the relative abundance ratio of protons to -particles (P/) at constant rigidity, this ratio is highly variable from one solar cosmic ray event to another. This ratio observed at the Earth, however, decreases monotonically with time from the onset of solar flares and, furthermore, is dependent on the heliocentric distance of the parent flares from the central meridian of the solar disk. P/'s which have been measured before the onset of SC geomagnetic storms change from 1.5 to 50 or more, being a function of the westward position of the source from the east limb of the Sun. These variations with respect to time and heliocentric distance suggest that the propagation of solar cosmic rays is strongly modulated in the interplanetary space. The major part of the -particles seem to propagate as if they are trapped within the magnetic clouds which produce SC geomagnetic and cosmic ray storms at the earth.The chemical composition and rigidity spectra of solar cosmic rays suggest that solar cosmic rays are mainly accelerated by the Fermi mechanism in solar flares. The observed variation of P/'s is produced mainly through the difference between the propagation characteristics of protons and -particles.NAS-NRC Associate with NASA.     

Solar flare nomenclature

The evolution of solar flare nomenclature is reviewed in the context of the paradigm shift, in progress, from flares to coronal mass ejections (CMEs) in solar-terrestrial physics. Emphasis is placed on: the distinction between eruptive (Class II) flares and confined (Class I) flares; and the underlying similarity of eruptive flares inside (two-ribbon flares) and outside (flare-like brightenings accompanying disappearing filaments) of active regions. A list of research questions/problems raised, or brought into focus, by the new paradigm is suggested; in general, these questions bear on the interrelationships and associations of the two classes (or phases) of flares. Terms such as eruptive flare and eruption (defined to encompass both the CME and its associated eruptive flare) may be useful as nominal links between opposing viewpoints in the flares vs CMEs controversy.     

New observational results for the solar chromosphere

High resolution photographs of the solar chromosphere have been obtained with the 40-cm refractor of the Athens Observatory and a 0.5 Å Halle H filter. Our best photographs show a resolution of 0.6, which is comparable with the theoretical resolving limit of 0.4, at H. The achieved resolution permitted us to secure some excellent observations of the fine structure of the chromosphere on the disk as well as on the limb. The study of these observations leads to the following results: (a) the bright filaments of the disturbed chromosphere as well as the penumbral ones appear to consist of knots, (b) inside the cells of the chromospheric network of the quiet chromosphere, bright roundish granule-like formations are present, their mean size being of the order of 2500 km. (c) the bright fine mottles seem to lie at the root of the elongated dark ones, each pair of them giving rise to a spicule.     

Velocity waves in the quiet solar chromosphere

Propagation of velocity waves are investigated in the solar chromosphere, with a special view to high frequencies (periods 60 s). Four line profiles have been observed during 27 mn with the Sacramento Peak vacuum telescope (H, 3933, 8498 and 8542 Ca ii). Three Fourier analysis are performed according to the location in the cells of the chromospheric network. Phase-shifts and amplitude ratios between the line Doppler shifts are computed as functions of frequency. The pollution of high frequency results by energetic low frequency oscillations is investigated.H Doppler shifts are probably affected by the large width of line formation layers (low transfer function). Using formation altitudes for Doppler shifts previously computed for the infra-red lines, we show that acoustic waves propagating upwards cannot account for the observations. In particular, the phase-shifts between oscillations in different chromospheric layers are much smaller than theoretical predictions. As a first attempt for a qualitative agreement, we suggest that most of the high frequency oscillations (10–15 mHz) are magnetoacoustic waves, travelling in layers where the gradient of the Alfvén-speed cannot be neglected, and reflected at the top of the chromosphere. The amplitudes of these waves are probably underestimated as derived from the observed Doppler shifts.     

Excitation of whistler waves driven by an electron temperature anisotropy

A 3-D particle simulation of excitation of whistler waves driven by an electron temperature anisotropy (T > T ) is presented. Results show that whistler waves can have appreciable growth driven by the anisotropy. The maximum intensity of the excited whistler waves increases as a quadratic function of the anisotropy. Due to the presence of a threshold, one needs a relatively large electron temperature anisotropy above threshold to generate large-amplitude whistler waves. The average amplitude of turbulence in the context of whistler waves is up to as large as about 1% of the ambient magnetic field when T /T . The total energy density of the whistler turbulence is adequate for production of relativistic electrons in solar flares through stochastic acceleration.